This invention relates generally to an electrophotographic printing machine, and more particularly concerns a sheet transport for moving a sheet in a path to enable a toner image to be transferred thereto. The invention also particularly concerns a sheet transport for moving a sheet in a recirculating path to enable successive toner powder images to be transferred thereto in superimposed registration with one another.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner into contact therewith. This forms a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
Some systems which have been designed for transporting a copy sheet into registration with a toner image developed on a moving member accelerate the copy sheet during transfer of the toner image from the moving member to the copy sheet. Such acceleration may occur when the leading portion of the sheet is being negotiated through a nonlinear path while at the same time the trailing portion of the copy sheet is traveling through the transfer zone. The above acceleration may cause a deterioration of the integrity of the image produced on the copy sheet due to slip between the copy sheet and the moving member while the sheet is traveling through the transfer zone. An example of the above deterioration is a blurred or smeared image produced on the copy sheet.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 4,118,025 PA0 Patentee: Konars et al. PA0 Issued: Oct. 3, 1978 PA0 U.S. Pat. No. 4,441,390 PA0 Patentee: Hechler et al. PA0 Issued: Apr. 10, 1984 PA0 U.S. Pat. No. 4,697,512 PA0 Patentee: Simeth PA0 Issued: Oct. 6, 1987 PA0 U.S. Pat. No. 4,849,795 PA0 Patentee: Spehrley, Jr. et al. PA0 Issued: Jul. 18, 1989 PA0 U.S. Pat. No. 4,905,052 PA0 Patentee: Cassano et al. PA0 Issued: Feb. 27, 1990
The relevant portions of the foregoing disclosures may be briefly summarized as follows:
U.S. Pat. No. 4,118,025 discloses a document conveying apparatus having a plurality of equally spaced gripping members. As the document is fed to the apparatus, the leading edge of the document is gripped between two gripping members and thereafter transported to a desired location.
U.S. Pat. No. 4,441,390 describes a sheet separating and transport apparatus in which tear-off rollers gently grip sheets. A pair of belts are provided which are positionable so as to grip the leading edge of a sheet as it is being fed by a conveyor belt.
U.S. Pat. No. 4,697,512 discloses a sheet gripper system having regular sheet grippers with additional sheet grippers provided in spaces between the regular grippers. The additional grippers are provided so that the front edge of the sheet is held by approximately twice the number of grippers before it enters the printing area, thereby reducing the tensile stress on the sheet as it passes through the printing zone by at least approximately half.
U.S. Pat. No. 4,849,795 describes an apparatus for moving a sheet in a recirculating path by spaced belts having a sheet gripper. The leading edge of the sheet is received by the gripper securing the sheet thereto for movement in a recirculation path. The belts move the sheet into contact with a photoconductive member in a transfer zone in synchronism with a toner image developed thereon.
U.S. Pat. No. 4,905,052 discloses a sheet transport velocity mismatch apparatus. A plate, interposed between adjacent sheet transports, supports the sheet until the leading edge thereof advances from the first sheet transport to the second sheet transport. When the leading edge of the sheet is received by the second sheet transport, the plate pivots away from the sheet to a location remote therefrom. Since the first sheet transport advances the sheet at a greater velocity than the second sheet transport, the sheet forms a buckle to compensate for velocity mismatch between the sheet transports.